A very simple model system, the cellular slime mold Dictyostelium discoideum, is being used to study mechanisms which control developmental gene activation during normal differentiation. Post-aggregation Dictyostelium cells transcribe an additional 26% of their genome which is not expressed in earlier pre-aggregation stage cells. Our previous studies have indicated that cell-cell interaction is a necessary prerequisite for the synthesis and stability of the late messenger RNAs. Following activation the actual rate of transcription and the subsequent stability of many of the messenger RNAs transcribed off of this portion of the genome are further regulated by a cyclic AMP-mediated process. We are currently utilizing both cDNA and genomic clones of individual members of this group of late genes to explore basic aspects of their sequence and structural organization in chromatin with the long-term objective of understanding the nucleoprotein interactions which are important in the activation of their expression and subsequent regulation by cyclic AMP.